Recently, the rapid development of smaller, lighter, and higher performance communication and other electronic equipment has required the development of high performance and large capacity batteries to power such equipment. The demands for large capacity batteries have fostered investigation of rechargeable lithium batteries. Positive active materials for rechargeable lithium batteries use lithium-transition metal oxides and negative active materials use crystalline or amorphous carbonaceous materials or carbon composites. The active materials are coated on a current collector with a predetermined thickness and length, or they are formed as a film to produce electrodes. The electrodes together with a separator are wound to produce an electrode element and the electrode element is inserted into a battery case such as a can followed by insertion of an electrolyte to fabricate a battery.
The rechargeable lithium battery theoretically exhibits an average discharge voltage of about 3.6 to 3.7V, which is higher than other alkaline batteries such as a Ni-MH (M is a hydrogen storage metal) battery or a Ni—Cd battery. However, such a high discharge voltage can be obtained only from an electrolyte which is electrochemically stable at charge and discharge voltage, 0 to 4.2V. The electrochemically stable electrolyte includes non-aqueous mixed carbonates such as ethylene carbonate or dimethyl carbonate.
During initial charging, lithium ions from a lithium-transition metal oxide positive electrode transfer to a carbonaceous negative electrode to cause the intercalation of lithium ions to the carbonaceous negative electrode. At this time, highly reactive lithium reacts with the carbonaceous negative electrode to generate Li2CO3, LiO, or LiOH, thereby forming a thin film on a surface of the negative electrode. Such a thin film is called a solid electrolyte interface (SEI) film. The SEI film not only prevents side reactions between lithium ions and a carbonaceous negative electrode or other material during charging and discharging, but also acts as an ion tunnel, allowing the passage of only lithium ions. The ion tunnel prevents the disintegration of the structure of the carbonaceous negative electrode because organic solvents in an electrolyte with a high molecular weight solvate lithium ions, and the solvent and the solvated lithium ions are co-intercalated into the carbonaceous negative electrode. Once the SEI film is formed, side reactions are inhibited, maintaining the amount of lithium ions. That is, the carbonaceous negative electrode reacts with an electrolyte during the initial charging to form a passivation layer such as an SEI film on the surface of the negative electrode, thereby preventing the decomposition of the electrolyte and allowing stable charging and discharging (J. Power Sources, 51(1994), 79–104). According to the mechanism, an irreversible formation reaction of the passivation layer occurs during the initial charging and discharging and does not occur thereafter, thereby providing a battery with stable cycle life characteristics.
However, thin prismatic batteries are problematic in that the carbonate-based organic solvent of the electrolyte can decompose to generate gases in the battery (J. Power Sources, 72(1998), 66–70). These gases include H2, CO, CO2, CH4, CH2, C2H6, C3H8, C3H6, etc. depending on the type of non-aqueous organic solvent and negative active material used. Such generated gas causes expansion of the volume of the battery and an increase in electrochemical energy and heat energy when the battery is stored at high temperatures, thereby slowly disintegrating the passivation layer which results in a side reaction between an exposed surface of the negative electrode. In addition, the generation of gas causes an increase in internal pressure, which induces the deformation of prismatic batteries and lithium polymer batteries, thereby deteriorating battery performance and stability.
One attempt to solve these problems has been to add an additive to an electrolyte. As the additive, carbonate-based compounds are disclosed in U.S. Pat. No. 5,626,981 and Japanese Laid-Open Patent No. 2002-15769. However, there are various problems with these methods: the added compound is decomposed or forms an unstable film by interacting with the carbon negative electrode during initial charging and discharging according to inherent electrochemical characteristics, resulting in the deterioration of the ion mobility in an electrode; and gas is generated inside the battery such that there is an increase in internal pressure, resulting in the significant worsening of the storage characteristics, stability, cycle life, and capacity of the battery.
Toshiba has attempted to use γ-butyrolactone in pouch-type batteries in order to suppress swelling (expansion of the battery), but it has not been effective.